Abstract:

The present invention relates to new water-dilutable or water-soluble
blocked polyisocyanates which allow the preparation of bakeable
one-component (1K) polyurethane coating materials which exhibit rapid
initial physical drying, exhibit reduced thermal yellowing and lead to
haze-free coatings, to a process for preparing them, and to their use.

Claims:

1. Process for preparing aqueous dispersions of mixedly blocked
polyisocyanate prepolymers, comprising:1) preparing a polyisocyanate
prepolymer by reacting:a) 100 equivalents of at least one polyisocyanate
component,b) 10 to 75 equivalents of one or more lactams, as blocking
agent(s) for isocyanate groups,c) 2 to 50 equivalents of blocking agents
for isocyanate groups, other than b),d) 0 to 15 equivalents of at least
one nonionic hydrophilicizing agent containing isocyanate-reactive
groups,e) 0.5 to 13 equivalents of at least one (potentially) anionic
hydrophilicizing agent containing isocyanate-reactive groups,f) 0 to 30
equivalents of one or more amino-free compounds having a molecular weight
of from 62 to 250 g/mol and which have either 2 to 4 OH groups or at
least one OH group and at least one further isocyanate-reactive group,
andg) 0 to 30 equivalents of one or more (cyclo)aliphatic compounds
having a molecular weight of from 32 to 300 g/mol and which have either 2
to 4 amino groups or at least one amino group and at least one further
isocyanate-reactive group,2) dissolving or dispersing the polyisocyanate
prepolymer in water during or after reaction of components a) to g) with
one another, and3) at least partially deprotonating the potentially
anionic groups of the hydrophilicizing agents used in e) with a base
before, during or after step 2).

2. Process for preparing aqueous dispersions of mixedly blocked
polyisocyanate prepolymers according to claim 1, wherein polyisocyanates
based on hexamethylene diisocyanate, isophorone diisocyanate and/or
4,4'-diisocyanatodicyclohexylmethane are used in component a).

3. Process for preparing aqueous dispersions of mixedly blocked
polyisocyanate prepolymers according to claim 1, wherein
ε-caprolactam is used as a blocking agent in component b).

[0002]The present invention relates to new water-dilutable or
water-soluble blocked polyisocyanates which allow the preparation of
bakeable one-component (1K) polyurethane coating materials which exhibit
rapid initial physical drying, exhibit reduced thermal yellowing and lead
to haze-free coatings, to a process for preparing them, and to their use.

[0003]In the coating of substrates there is an increasing use of aqueous
binders, especially polyurethane (PU) dispersions with blocked isocyanate
groups. The preparation of aqueous PU dispersions and the methods of
coating and of baking are known.

[0004]Aqueous one-component polyurethane baking varnishes whose
crosslinker component is composed substantially of blocked
polyisocyanates (BNCO crosslinkers, crosslinker dispersions), however,
exhibit slow initial physical drying following application of the
coating. This leads to problems during transport of the coated articles
to the baking oven in which the chemical crosslinking takes place. The
articles, for example, may stick to conveyor belts or gloves. A long
period of drying is therefore necessary prior to the baking operation.

[0006]A further problem associated with the processing of one-component
polyurethane baking varnishes with blocked isocyanate groups is the
hazing of the coating film, which poses a particular hindrance to the
coating of transparent substrates (such as glass) for the purpose of
obtaining transparent coated systems.

[0007]EP-A 0802210 describes water-dilutable polyisocyanate crosslinkers
with blocked isocyanate groups. To circumvent the problem of thermal
yellowing, the use of compounds carrying hydrazide groups is proposed.
The coating of glass in accordance with EP-A 0807650 using polyisocyanate
crosslinkers of this kind does lead to clear, unyellowed films, but the
initial physical drying behaviour of the systems is very slow and hence
disadvantageous.

[0008]The mixed blocking of polyisocyanates with lactams and with other
blocking agents is well known from the field of the non-aqueous
polyurethane systems and is described in, for example, DE-A 10156897,
DE-A 4416750, EP-A 0403044, DE-A 3128743 and U.S. Pat. No. 5,455,374.
Conclusions of reduced yellowing or of advantageous initial drying
behaviour, of aqueous PU systems in particular, through the use of
lactam-based mixed blocking, however, are not possible.

[0009]It was an object of the present invention, then, to provide
water-dilutable or water-soluble blocked polyisocyanates which in the
form of an aqueous dispersion, after blending with polyol components,
lead to bakeable one-component polyurethane coating materials with rapid
initial physical drying after application and with low thermal yellowing
on baking, or even on overbaking. A further object was to enable
haze-free coating of substrates with the resultant coating composition.

[0010]It has now surprisingly been found that hydrophilicized
polyisocyanates which exhibit mixed blocking with a lactam and a further
blocking agent meet these requirements.

[0012]1) preparing a polyisocyanate prepolymer by reacting: [0013]a) 100
equivalents of at least one polyisocyanate component, [0014]b) 10 to 75
equivalents of one or more lactams, as blocking agent(s) for isocyanate
groups, [0015]c) 2 to 50 equivalents of further blocking agents for
isocyanate groups, other than b), [0016]d) 0 to 15 equivalents of at
least one nonionic hydrophilicizing agent containing isocyanate-reactive
groups, [0017]e) 0.5 to 13 equivalents of at least one (potentially)
anionic hydrophilicizing agent containing isocyanate-reactive groups,
[0018]f) 0 to 30 equivalents of one or more amino-free compounds of the
molecular weight range from 62 to 250 g/mol which have either 2 to 4 OH
groups or at least one OH group and at least one further
isocyanate-reactive group, and [0019]g) 0 to 30 equivalents of one or
more (cyclo)aliphatic compounds of the molecular weight range from 32 to
300 g/mol which have either 2 to 4 amino groups or at least one amino
group and at least one further isocyanate-reactive group,

[0020]2) dissolving or dispersing the polyisocyanate prepolymer in water
during or after reaction of components a) to g) with one another, and

[0021]3) at least partially deprotanating the potentially anionic groups
of the hydrophilicizing agents used in e) with a base before, during or
after step 2).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022]The amounts of the components b) to g) in equivalents refer to the
respective amounts of isocyanate-reactive groups of the compounds
contained in these components, the isocyanate component used in a) having
100 equivalents of free NCO groups available for reaction.

[0023]In one preferred embodiment, the reactants a) to f) are reacted with
one another and then dispersed or dissolved in water, this step being
accompanied or followed by the at least partial deprotonation of the
potentially anionic groups of the hydrophilicizing agents used in e) with
a base. The optionally added component g) is preferably added after the
prepolymer has been dispersed in water.

[0024]In one particularly preferred embodiment, component a) is reacted
first of all with components d), e) and f), and this reaction is followed
by reaction with component b) and then with component c). Subsequently a
base is added for deprotonation and the reaction mixture is dispersed in
water. Finally it is possible to add component g).

[0025]The proportions of the reactants are preferably selected such that
the equivalent ratio of the isocyanate component a) to
isocyanate-reactive groups of components b), c), d), f) and g) is 1:0.7
to 1:1.3, preferably 1:0.85 to 1:1.1. The calculation of this equivalent
ratio is made with exclusion not only of the acid groups of component e)
but also of the solvent or water used to prepare solutions or dispersions
of the polyurethanes, and also of the deprotonating agent used to
deprotonate the acid groups.

[0026]Suitable polyisocyanates used in component a) are the NCO-functional
compounds that are known per se to a person skilled in the art and have a
functionality of preferably 2 or more. These are typically aliphatic,
cycloaliphatic, araliphatic and/or aromatic di- or triisocyanates and
also their higher molecular weight derivatives having
iminooxadiazinedione, isocyanurate, uretdione, urethane, allophanate,
biuret, urea, oxadiazinetrione, oxazolidinone, acylurea and/or
carbodiimide structures, and containing two or more free NCO groups.

[0028]Such polyisocyanates typically have isocyanate contents of 0.5% to
60%, preferably 3% to 30%, more preferably 5% to 25% by weight.

[0029]In the process of the invention it is preferred to use the
relatively high molecular weight compounds having isocyanurate, urethane,
allophanate, biuret, iminooxadiazinetrione, oxadiazinetrione and/or
uretdione groups that are based on aliphatic and/or cycloaliphatic
diisocyanates.

[0030]In the process of the invention it is particularly preferred to use,
in component a), compounds having biuret, iminooxadiazinedione,
isocyanurate and/or uretdione groups that are based on hexamethylene
diisocyanate, isophorone diisocyanate and/or
4,4'-diisocyanatocyclohexylmethane.

[0031]Very particular preference is given to using polyisocyanates with an
isocyanurate structure that are based on isophorone diisocyanate.

[0032]Blocking agents suitable as component b) are lactams (cyclic amides)
which possess an amidic H atom. Examples are λ-butyrolactam
(2-pyrrolidone), δ-valerolactam and/or ε-caprolactam;
ε-caprolactam is preferred. Component b) is used in an amount of
preferably 30 to 65 equivalents, based on the NCO groups of the
isocyanate component a).

[0033]As component c) it is possible to use the monofunctional blocking
agents which are known per se in the art for the blocking of isocyanate
groups and which are not contained in component b). Examples are phenols,
oximes, such as butanone oxime, acetone oxime or cyclohexanone oxime,
amines such as N-tert-butylbenzylamine or diisopropylamine,
3,5-dimethylpyrazole, triazole, esters containing deprotonatable groups,
such as diethyl malonate, ethyl acetoacetate, and mixtures thereof and/or
mixtures with other blocking agents. Preference is given to butanone
oxime, acetone oxime, 3,5-dimethylpyrazole and/or mixtures thereof,
particular preference to butanone oxime. Component c) is used in an
amount of preferably 10 to 30 equivalents, based on the NCO groups of
isocyanate component a).

[0034]The hydrophilicizing component d) is composed of at least one
nonionically hydrophilicizing compound which contains isocyanate-reactive
groups. Examples of these compounds include polyoxyalkylene ethers which
contain at least one hydroxyl or amino group and also one or more
oxyalkylene units, of which at least one is an oxyethylene unit. These
polyoxyalkylene ethers are obtainable in conventional manner by
alkoxylation of suitable starter molecules.

[0036]Alkylene oxides suitable for the alkoxylation reaction are, in
particular, ethylene oxide and propylene oxide, which can be used in any
order or else in a mixture for the alkoxylation reaction. Preference is
given to the blockwise addition of ethylene oxide and propylene oxide
with the starter.

[0038]The amount of ethylene oxide units in relation to the total solids
content of components a) to g) is below 30%, preferably below 20%, more
preferably below 15% by weight.

[0039]Component d) is used in an amount of preferably 3 to 8 equivalents,
based on the NCO groups of the isocyanate component a).

[0040]The hydrophilicizing component e) is composed of at least one
(potentially) anionic compound having at least one group that is reactive
towards isocyanate groups.

[0041]These compounds are preferably carboxylic acids having at least one,
preferably one or two, hydroxyl groups, or salts of such
hydroxycarboxylic acids. Examples of suitable such acids include
2,2-bis(hydroxymethyl)alkanecarboxylic acids such as dimethylolacetic
acid, 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid or
2,2-dimethylolpentanoic acid, dihydroxysuccinic acid, hydroxypivalic acid
or mixtures of such acids.

[0042]As component e) it is preferred to use dimethylolpropionic acid
and/or hydroxypivalic acid.

[0044]Component e) is used in an amount of preferably 5 to 9 equivalents,
based on the NCO groups of the isocyanate component a). Groups defined as
isocyanate-reactive groups in this case are the alcohol groups; the
carboxylic acid groups and/or carboxylate groups are not rated as
isocyanate-reactive groups.

[0048]Preference is given to using in f) compounds of the aforementioned
kind having molecular weights of 62 to 200 g/mol.

[0049]Component f) is used in an amount of preferably 3 to 15 equivalents,
based on the NCO groups of the isocyanate component a).

[0050]As chain extender component g) it is possible to use
isocyanate-reactive organic diamines or polyamines such as
1,2-ethylenediamine, 1,2- and 1,3-diamino-propane, 1,4-diaminobutane,
1,6-diaminohexane, isophoronediamine, an isomer mixture of 2,2,4- and
2,4,4-trimethylhexamethylenediamine, 2-methylpentamethylenediamine,
diethylenetriamine, 4,4-diamino-dicyclohexylmethane, and/or
dimethylethylenediamine

[0051]As component g) it is also possible, moreover, to use compounds
which as well as a primary amino group also have secondary amino groups,
or which as well as an amino group (primary or secondary) also have OH
groups or SH groups.

[0052]Examples of such compounds are primary/secondary amines, such as
3-amino-1-methylaminopropane, 3-amino-1-ethylaminopropane,
3-amino-1-cyclohexyl-aminopropane, 3-amino-1-methylaminobutane,
alkanolamines such as N-aminoethylethanolamine, ethanolamine,
diethanolamine, 3-aminopropanol, 1-aminopropanol, neopentanolamine,
N-methylethanolamine and/or N-methyl-isopropanolamine and alkanethiol
amines, such as 1-aminopropanethiol.

[0053]As component g) it is preferred to use diamines or polyamines, such
as ethylenediamine, isophoronediamine, 1,6-diaminohexane and/or
4,4-diaminodicyclohexylmethane.

[0054]In component g) it is preferred to use compounds of the
aforementioned kind having molecular weights of 60 to 300 g/mol.

[0055]Component g) is used in an amount of preferably 0 to 10 equivalents,
based on the NCO groups of the isocyanate component a).

[0056]For chain extension it is also possible to use mixtures of
components f) and g).

[0057]Besides chain extension by reaction with components f) and/or g) it
is also possible for free NCO groups to react with water, in the course
of dispersion, for example, with formation of amine, the primary amino
groups thus fowled being consumed by reaction in turn with free NCO
groups, with accompanying chain extension.

[0058]In the preparation of the dispersion of the invention it is also
possible to use solvents and/or for the raw materials to be used as
solutions. Examples of suitable solvents are N-methylpyrrolidone,
N-ethylpyrrolidone, xylene, toluene, butyl acetate, methoxypropyl
acetate, acetone or methyl ethyl ketone.

[0059]Where volatile, (partly) water-miscible solvents such as acetone or
methyl ethyl ketone are used they are typically separated off by
distillation following dispersion in water. This procedure is also termed
the acetone process or slurry process. The advantage lies in a reduced
viscosity for the preparation of the prepolymer, without the solvent
still being present in the completed dispersion.

[0060]A further possibility is to add solvent after the consumption of the
isocyanate groups by reaction. In this case it is also possible to employ
protic solvents such as alcohols, which serve for example to stabilize
the dispersion or to improve coating-material properties.

[0061]The amount of the water that is used as the dispersing medium is
generally made such that the resulting dispersions are 20% to 60% by
weight dispersions, preferably 30% to 45% by weight dispersions, based on
solids content in water.

[0062]Examples of deprotonating agents for converting the potentially
anionic groups into their anionic form are basic compounds such as sodium
hydroxide, potassium hydroxide, ammonia, primary or secondary amines,
such as diisopropanolamine or 2-amino-2-methyl-1-propanol, tertiary
amines such as triethylamine, dimethyl-cyclohexylamine,
diisopropylcyclohexylamine, diisopropylethylamine, triethanolamine,
methyldiethanolamine, N,N-dimethylaminoethanol, or N-methylmorpholine, or
any desired mixtures thereof.

[0063]Preferred deprotonating agents are tertiary amines such as
triethylamine, diisopropylethylamine and N,N-dimethylethanolamine;
N,N-dimethylethanolamine is particularly preferred.

[0064]The amount of deprotonating agent used is generally made such that
the degree of deprotonation of the carboxylic acid groups present in the
polyurethanes of the invention (molar ratio of amine/hydroxide employed
to acid groups present) is at least 40%, preferably 70% to 130%, more
preferably 90% to 110%. This deprotonation may take place before, during
or after the dispersing or dissolving step. Preference is nevertheless
given to deprotonation prior to the addition of water.

[0065]To accelerate the urethanization during prepolymer preparation, a
further possibility is to add catalysts to the reaction mixture. Examples
of suitable catalysts include tertiary amines, tin compounds, zinc
compounds or bismuth compounds, or basic salts. Those preferred are
dibutyltin dilaurate and dibutyltin dioctoate.

[0066]The invention further provides the aqueous dispersions of mixedly
blocked polyisocyanate prepolymers that are obtained by the
above-described process, and also the prepolymers contained therein
themselves.

[0074]The dispersions of the invention and also the mixedly blocked
prepolymers of the invention can be used for producing aqueous, bakeable
coating compositions (baking varnishes), for the coating of substrates,
preferably of metals, minerals, wood, plastics, for industrial coating
for example, glass, in textile coating and in automotive OEM finishing.

[0075]Additionally provided by the invention are the use of the
dispersions of mixedly blocked polyisocyanate prepolymers of the
invention in the preparation of coating compositions, and also the
resultant coating compositions and coatings themselves, and the
substrates provided with such coatings.

[0076]For the preparation of coating compositions of this kind, the
dispersions of the invention are typically blended with water-soluble or
-dispersible polyhydroxy compounds and optionally auxiliaries and
adjuvants.

[0077]Suitable polyhydroxyl compounds for this end use and also further
details relating to the preparation and application of such baking
varnishes are known. They are preferably the conventional aqueous or
water-thinnable binders based on polyhydroxy polyesters, polyhydroxy
polyurethanes, polyhydroxy polyethers, polycarbonate diols or
hydroxyl-containing polymers, such as the conventional polyhydroxy
polyacrylates, polyacrylate polyurethanes and/or polyurethane
polyacrylates.

[0078]They are typically hydrophilically modified, as described for
example in EP-A-0 157 291, EP-A-0 498 156 or EP-A-0 427 028.

[0079]Such polyhydroxyl compounds generally have a hydroxyl number of 20
to 200, preferably of 50 to 130 mg KOH/g.

[0080]In the coating compositions of the invention it is possible, in
addition to the inventively essential dispersions, to use other
alcohol-reactive compounds as well, such as amino crosslinker resins such
as melamine resins and/or urea resins for additional crosslinking on
baking. Likewise possible is the use of further hydrophilic
polyisocyanates.

[0081]Auxiliaries and adjuvants that can be added are the substances that
are typical per se, such as pigments, fillers, flow control agents,
defoamers and catalysts. To improve coating-material adhesion it is
possible for the coating materials to include commercially customary
additives such as, for example, mercaptosilanes such as
3-mercaptopropyltrimethoxysilane, epoxyalkylsilanes such as
3-glycidyloxypropyltriethoxysilane, aminoalkylsilanes such as
3-aminopropyltriethoxysilane, their hydrolysis products, or mixtures of
these components.

[0082]The coating compositions of the invention are prepared by methods
which are known per se.

[0083]For the purpose of coating it is possible for the coating
compositions of the invention to be applied by knife coating, dipping, by
spray application such as compressed-air spraying or airless spraying,
and also by electrostatic application, as for example high-speed rotating
bell application. The dry film thickness may for example be 10 to 120
μm. The dried films are cured by baking in the temperature range from
90 to 200° C., preferably 130 to 190° C., more preferably
140 to 180° C. Curing under the influence of microwave radiation
is also possible. Baking may be preceded by physical drying of the film,
for example at temperatures between 20 and 90° C.

[0107]At 70° C. in a standard stirred apparatus with nitrogen
blanketing, 359.0 g of Desmodur® Z 4470 M/X were admixed in
succession with a solution of 4.7 g of dimethylolpropionic acid in 9.4 g
of N-methylpyrrolidone (NMP), 37.5 g of Carboxwax® 750 and 3.39 g of
neopentyl glycol. The batch was then heated to 80° C. and stirred
until a constant NCO value of 8.02% (calculated: 8.27%) was reached. It
was cooled to 70° C. Then 71.0 g of butanone oxime were added at a
rate such that the temperature in the reaction vessel did not exceed
80° C. This was followed by further stirring at 80° C.
until NCO groups were no longer detectable by IR spectroscopy, then by
cooling to 70° C. and addition of 2.50 g of dimethylethanolamine.
After further cooling to 60° C., the batch was dispersed with
570.8 g of deionized water at 25° C. It was conditioned at
50° C., stirred for 1 hour and left to cool to room temperature
with stirring.

[0110]At 70° C. in a standard stirred apparatus with nitrogen
blanketing, 466.66 g of Desmodur® Z 4470 M/X were admixed with a
solution of 13.1 g of dimethylolpropionic acid in 26.2 g of
N-methylpyrrolidone (NMP). The batch was then heated to 80° C. and
stirred until a constant NCO value of 9.12% (calculated: 9.17%) was
reached. Then 48.75 g of Carbowax® 750 were added and the mixture was
stirred at 75° C. until an NCO value of 7.84 (calculated: 7.87%)
was reached. It was then cooled to 70° C. and thereafter 76.0 g of
acetone oxime were added at a rate such that the temperature in the
reaction vessel did not exceed 80° C. This was followed by further
stirring at 80° C. until NCO groups were no longer detectable by
IR spectroscopy, then by cooling to 70° C. and addition of 8.70 g
of dimethylethanolamine. After further cooling to 60° C., the
batch was dispersed with 1548 g of deionized water at 25° C. It
was conditioned at 50° C., stirred for 1 hour and left to cool to
room temperature with stirring.

[0112]Not Inventive, Preparation of a Crosslinker Dispersion Blocked with
Caprolactam

[0113]At 70° C. in a standard stirred apparatus with nitrogen
blanketing, 359.0 g of Desmodur® Z 4470 M/X were admixed in
succession with a solution of 4.7 g of dimethylolpropionic acid in 9.4 g
of N-methylpyrrolidone (NMP), 37.5 g of Carboxwax® 750 and 3.39 g of
neopentyl glycol. The batch was then heated to 80° C. and stirred
until a constant NCO value of 8.02% (calculated: 8.27%) was reached. It
was cooled to 70° C. and then 92.3 g of ε-caprolactam were
added. This was followed by further stirring at 100° C. until NCO
groups were no longer detectable by IR spectroscopy, then by cooling to
70° C. and addition of 2.50 g of dimethylethanolamine. After
further cooling to 60° C., the batch was dispersed with 770.53 g
of deionized water at 25° C. It was conditioned at 50° C.,
stirred for 1 hour and left to cool to room temperature with stirring.

[0116]At 70° C. in a standard stirred apparatus with nitrogen
blanketing, 359.0 g of Desmodur® Z 4470 M/X were admixed in
succession with a solution of 4.7 g of dimethylolpropionic acid in 9.4 g
of N-methylpyrrolidone (NMP), 37.5 g of Carbowax® 750 and 3.39 g of
neopentyl glycol. The batch was then heated to 80° C. and stirred
until a constant NCO value of 8.24% (calculated: 8.27%) was reached. It
was cooled to 70° C. and then 63.4 g of ε-caprolactam were
added, Stirring was continued at 100° C. until a constant NCO
value of 1.75% (calculated: 1.76%) was reached, followed by cooling to
75° C. and addition of 17.4 g of butanone oxime. Stirring was
continued until NCO groups were no longer detectable by IR spectroscopy,
then by cooling to 70° C. and addition of 2.50 g of
dimethylethanolamine. After further cooling to 60° C., the batch
was dispersed with 582 g of deionized water at 25° C. It was
conditioned at 50° C., stirred for 1 hour and left to cool to room
temperature with stirring.

[0130]For determination of the performance data, the blended coating
materials of Examples 9 to 18 were formulated in accordance with Table 1,
applied and cured. The performance data are contained in Table 2.

[0131]Table 1

[0132]Clearcoat materials 9 to 18 were prepared by mixing the blocked
polyisocyanates with the polyol component in the ratio of their
equivalent weights (BNCO:OH 1:1). To improve their adhesion, the coating
materials include commercially customary additives (1.1%, calculated on
the basis of solid binder).

[0133]The above clearcoat materials were applied to glass plates 3 mm
thick, from Schlier & Hermes, using a coating knife from Deka (No. 120)
and were baked in a forced-air oven at 170° C. for 30 minutes.
This gave dry film thicknesses of approximately 25-30 μm.

[0137]a) Solvent Resistance [0138]To determine the solvent resistance, a
cottonwool pad soaked with solvent was placed onto a coated substrate and
covered with a watch glass. After the exposure time, the cottonwool pad
and any solvent residues are removed and the surface of the coating
material is rated by inspection.

[0139]b) Sodium Hydroxide Resistance [0140]To determine the sodium
hydroxide resistance, the substrates under investigation were immersed
vertically halfway into a bath containing 5% strength aqueous sodium
hydroxide solution, covered and heated at 70° C. for 8 h.
Thereafter the plates were rinsed off with deionized water and rated by
inspection.

[0141]c) Initial Physical Drying [0142]For the investigation of the
initial physical drying, the corresponding clearcoat materials were
applied to 3 mm glass plates from Schlier & Hennes using a coating knife
from Deka (No. 120), subjected to preliminary drying at 80° C. for
3 minutes and then investigated for the absence of tack (0=tack-free to
5=highly tacky).

[0143]d) Film Hazing [0144]The coating films were inspected after baking
for signs of haze (0=nothing found to 5=very hazy)

[0145]Requirements with Regard to the Individual Technical Properties:

[0147]With the non-inventive self-crosslinking baking systems it is not
possible using the prior-art crosslinker dispersions to achieve a
sufficient profile of properties in respect of initial physical drying,
low thermal yellowing and absence of haze from the films (Examples 9-12).
Even the blend of two non-inventive crosslinker dispersions shown in
Example 13 (Example 1: oxime-blocked, Example 3: lactam-blocked) in the
preparation of a self-crosslinking dispersion did not result in
acceptable coatings.

[0148]Only when the inventive crosslinker dispersions were used, reacted
with a lactam and with a further blocking agent (Examples 14-18), is it
possible to achieve an optimum profile of the critical properties. The
other film properties as well, such as the hardness of the coating film,
solvent resistance and sodium hydroxide resistance, correspond to the
requirements imposed on a high-value coating system.

[0149]Although the invention has been described in detail in the foregoing
for the purpose of illustration, it is to be understood that such detail
is solely for that purpose and that variations can be made therein by
those skilled in the art without departing from the spirit and scope of
the invention except as it may be limited by the claims.